1. Field of the Invention
The present invention relates to a transmission method, a transmitter and a receiver for a multi antenna wireless communication system using cyclic delay diversity.
2. Description of the Related Art
Transmit antenna diversity that is one of transmit diversity techniques for wireless communication transmits the same signals from a plurality of antennas. As for the transmit antenna diversity, space time block coding (STBC) which makes transmission data blocks, operates its code, and changes the transmission order then transmits data; and cyclic delay diversity (CDD) which simultaneously transmits signals subjected to cyclic shift for blocks have been well known.
In the CCD, as described, for example, in G. Bauch and J. S. Malik, “Parameter optimization, interleaving and multiple access in OFDM with cyclic delay diversity,” VTC-2004 spring, Vol. 1, pp. 505-509 (2004) (hereinafter, referred to as Document 1), a transmitter transmits data signal from one antenna, and also it transmits the same data signal with the cyclic shift performed thereto from other antennas. In a receiver, the data signals transmitted from each antenna of the transmitter are mixed and received.
Cyclic-shifted signals have high velocity phase rotation in frequency domain. Therefore, mixing the cyclic-shifted signals with the not cyclic-shifted signals makes frequencies intensifying the signals mutually and frequencies weakening the signals mutually on a frequency domain at short frequency intervals. Thereby, the CCD eliminates burst power drop in the frequency directions. Therefore, if error correction coding has been implemented as well as transmission data has fully interleaved in the frequency directions, the CCD may fully exert error correction ability in the receiver, and may expect improving reception performance.
In the technique in the Document 1, the CCD requires an amplitude reference and phase reference for demodulating a spectrum varying with a high velocity in the frequency domain in order to demodulate received signals. The transmitter has to transmit pilot signals defied in a system to estimate channels from a plurality of antennas prior to the data signals.
Each pilot signal being a redundant signal not directly contributing to a data transmission, the use of the pilot signals with long time lengths causes a reduction in transmission efficiency of data. Therefore, a short pilot signal length (time length) is desired. However, although the Document 1 refers to a cyclic shift amount of the data signal, it does not refer to a cyclic shift amount of the pilot signal and the pilot signal length.
The object of the present invention is to shorten a pilot signal length as much as possible while enjoying an effect of the CDD to improve the data transmission efficiency.